An introduction in Bio-Corrosion and its prevention methods

Transcription

1 Port-said University Faculty of science Chemistry department An introduction in Bio-Corrosion and its prevention methods (Microbial-induced corrosion) Under supervision: Dr: Samir Abd-Elhady Prepared by: Amany Shalaby Abdo Shalaby Student in 4 th year Biochemistry department

2 Acknowledgment In the name of Allah,,,, Firstly, all thanks to Allah almighty for his generosity in completing this project. I wish to express my deepest gratitude to my parents, my sister and my brothers for their endless love, prayers and encouragement. I would like to express my special thanks and sincere gratitude to Dr/ Samir Abd-Elhady For his encouragement, valuable advice and for his constructive guidance that he kindly offered throughout the development of this research. And thanks to Dr/Ibrahim Mohiee, Head of Department of Chemistry. Finally, all my thanks to my friends for their encouragement and being beside me all the times. With my best wishes Amany Shalaby Abdo 2

3 Table of content Contents Introduction... 5 The importance of corrosion studies... 7 Definitions of corrosion Mechanism of corrosion: Corrosion of metals by non-electrolytes: Corrosion of metals in the presence of electrolytes: Potential ph Diagrams: Forms of Corrosion: General (Uniform) corrosion Galvanic corrosion Crevice corrosion Pitting corrosion Erosion corrosion Stress corrosion cracking selective leaching Liquid metal attack Filiform corrosion Biological Corrosion: Mechanism of biological corrosion Sulfate Reducing Bacteria (SRB) Iron Reducing Bacteria (IRB)

4 Prevention of biological corrosion: Selection and Modification of Environment Microbial Inhibitors Protective Coating: Cathodic Protection: Protective films Use of biocides Material change or modification and mechanical methods Corrosion Science and Corrosion Technology Reference. 56 4

5 Introduction Materials technology is a very vital part of modern technology. Technological development is often limited by the properties of materials and knowledge about them. Some properties, such as those determining corrosion behavior, are most difficult to map and to control. Corrosion is a disease to materials just like a disease to human beings. Some forms of corrosion can be prevented through good practices in materials selection and design, while others can be cured or controlled if diagnosed early. Corrosion diagnosis involves a number of destructive and non-destructive inspection and examination techniques such as visual inspection, chemical, electrochemical, mechanical, metallurgical, and microstructural tests and analyses. In a modern business environment, successful enterprises cannot tolerate major corrosion failures, especially those involving personal injuries, fatalities, unscheduled shutdowns and environmental contamination. For this reason considerable efforts are generally expended in corrosion control at the design stage and in the operational phase. This research discusses fundamentals of corrosion, Mechanism, and forms focuses on microbial-induced corrosion (MIC) of metallic materials as an introduction to the recognition, management, and prevention of microbiological corrosion. Ever since man began recovering metals from their ores and placing them in soil or aqueous environments, organisms have undoubtedly played a role in accelerating their corrosion. 5

6 In 1891, the possibility that microorganisms might exert an influence on the corrosion of metals was mentioned by Garrett [1]. He postulated that the increase in the corrosive action of lead could be due to the ammonia, nitrites, and nitrates produced by bacterial action. Gaines, in 1910, [2] suggested that the corrosion of iron in the soil and aqueous environments might be caused by sulfate-reducing, sulfur-oxidizing, and "iron" bacteria. The formation of deposits in water pipes by iron bacteria was reported by Ellis [3] and Harder [4] in Although corrosion was generally associated with the presence of oxygen, the process of anaerobic corrosion was encountered several times before Convincing evidence that corrosion took place in oxygen-free environments and that bacteria were responsible for it was provided in 1934 by von Wolzogen Ktihr and van der Vlugt [5]. They had observed severe corrosion of cast iron water pipes and mains in communities north of Amsterdam, requiring pipes to be replaced every 2-3 years. The highly anaerobic soil in which these pipes had been placed was primarily polder land, land which had been reclaimed from the sea. 6

7 The importance of corrosion studies: [6, 7] 1-Safety: Premature failure of bridges structures or Failure of operating equipment due to corrosion can result in human injury or even loss of life. Sudden failure can cause fire, explosion, release of toxic product, and construction collapse. 2-Conservation: Applied primarily to raw material metallic resources, the world s supply of which is limited, and the wastage of which includes corresponding losses of energy and water resources accompanying the production and fabrication of metal structures. Additional human energy and resources are also consumed in the replacement and redesign of corroded equipment and components. 3-Economic: Corrosion is not only dangerous, but also costly, with annual damages in the billions of dollars including the reduction of material losses resulting from the wasting away or sudden failure of piping, tanks, metal components of machines, ships, hulls, marine, structures etc. Economic losses can be divided into direct and indirect losses. 3.1 Direct losses are those losses associated with the direct replacement of corroded equipment, components, and structures. Also included are those costs, both of Labor and material, to maintain equipment and structures to prevent corrosion from taking place or to control the rate of corrosion. Falling into this category are such items as painting, application of protective coatings or linings, operating costs for cathodically protected pipelines and structures, and routine inspections or testing of equipment by online corrosion monitoring instruments. 7

8 Other examples of direct losses include the additional costs incurred by the use of corrosion-resistant metals or alloys instead of less-expensive carbon steel (when carbon steel has adequate mechanical properties but insufficient corrosion resistance), by the application of corrosion-resistant coatings to carbon steel, or by the addition of corrosion inhibitors to water. 3.2 Indirect Losses although the causes of indirect losses can be listed, it would be extremely difficult to place an actual cost on these losses. However, it would be safe to assume that these costs would be some multiple of the direct losses. Typical of these indirect losses are the following examples: Shutdown: Unplanned shutdowns because of the failure of equipment resulting from corrosion lead to loss of production and consequently loss of profit. Although the actual cost of maintenance work may be minimal, the value of the lost production can be considerable. If this type of occurrence is frequent, the cost is usually added to the cost of the product Loss of Product: Many times, corrosion is so severe that leakage will develop that permits loss of product. If this leakage occurs in a pipeline, it may go undetected for an extended period, during which time there is a continuous loss of product. For example, from a container that has corroded through. If the leaking material itself is a corrosive material, it will attack its surroundings, thus causing additional loss. There have also been cases where leakage from underground tanks, such as gasoline, has contaminated the soil and even in some cases made the water in wells unsuitable for use. 8

9 3.2.3 Contamination: During the corrosion of a metal, the fluid being transported, stored, processed, or packaged in a metallic component can pick up metallic salts. This metallic pickup can be detrimental to the product; with soap products a shortened shelf life, with dyes a color alteration, and in some cases of intermediate products the inability to carry out succeeding process steps. For many years, lead pipes were used to transport water until it was determined that the lead pick-up in the water caused lead poisoning in humans Environmental Damage: Corrosion of equipment used to control atmospheric pollution resulting from processing operations can result in a decrease in efficiency. Such a decrease permits pollutants from the manufacturing operation to enter the atmosphere Loss of Efficiency: Corrosion in a piping system can result in the buildup of a scale. This scale can cause a reduction in heat transfer as well as an increase in the power required to pump the fluid through the system. The efficient operation of other mechanical equipment can also be reduced by corrosion. This reduction in efficiency can cause an increase in operating costs as well as result in increased fuel consumption, lubricant loss, and reduced work output Overdesign: In many instances when the corrosive effect of the system is known, additional thicknesses of vessel shells will be provided for in the design. This is known as corrosion allowance. Because this thickness is in addition to that required for the design conditions, an extra cost is involved. In some instances, the actual corrosive effect is not known and consequently, for safety reasons, a much thicker shell results. 9

10 Still other consequences are social. These can involve the following issues: *Health: For example, pollution due to escaping product from corroded equipment or due to a corrosion product itself. *Depletion of natural resources: Including metals and the fuels used to manufacture them. *Appearance: As when corroded material is unpleasing to the eye. Of course, all the preceding social items have economic aspects also. Clearly, there are many reasons for wanting to avoid corrosion. 10

11 GDP: Gross Domestic Product Cost of corrosion Estimate for 2013 by G2MT Labs 11

12 Premature failure of bridges or structures due to corrosion can result in human injury or even loss of life. Corrosion can result in loss of products. 12

13 Definitions of corrosion: Corrosion can be defined in many ways[8]. Some definitions are very narrow and deal with a specific form of corrosion, while others are quite broad and cover many forms of deterioration. The word corrode is derived from the Latin corrodere, which means to gnaw to pieces. The general definition of corrode is to eat into or wear away gradually, as if by gnawing. For purposes here, corrosion can be defined as a chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. Corrosion is the deterioration or destruction of metals and alloys in the presence of an environment by chemical or electrochemical means. In simple terminology, corrosion processes involve reaction of metals with environmental species. As per IUPAC, Corrosion is an irreversible interfacial react ion of a material (metal, ceramic, polymer) with its environment which results in its consumption or dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered. Exclusively physical or mechanical processes such as melting and evaporation, abrasion or mechanical fracture are not included in the term "corrosion".with the knowledge of the role of various microorganisms present in soil and water bodies, the definition for corrosion need be further widened to include microbially influenced factors. 13

14 Corrosion is a natural process. Just like water flows to the lowest level, all natural processes tend toward the lowest possible energy states. Thus, for example, iron and steel have a natural tendency to combine with other chemical elements to return to their lowest energy states. In order to return to lower energy states, iron and steel frequently combine with oxygen and water, both of which are present in most natural environments, to form hydrated iron oxides (rust), similar in chemical composition to the original iron ore. The following Figure illustrates the corrosion life cycle of a steel product. All metallic materials consist of atoms having valiancy electrons which can be donated or shared. In a corrosive environment the components of the metallic material get ionized and the movement of the electrons sets up a galvanic or electrochemical cell which causes oxidation, reduction, dissolution or simple diffusion of elements. The metallurgical approach of corrosion of metals is in terms of the nature of the alloying characteristics, the phases existing and their inter-diffusion under different environmental conditions. 14

15 In fact, the process of corrosion is a complex phenomenon and it is difficult to predict the exclusive effect or the individual role involved by any one of the above mentioned processes. Based on the above processes, corrosion can be classified in many ways such as: Chemical and electrochemical. High temperature and low temperature. Wet corrosion and dry corrosion. Chemical corrosion in which the metal is converted into its oxide when the metal is exposed to a reactive gas or non-conducting liquids. Electrochemical corrosion the formation of hydrous oxide film occurs when the metal is immersed in a conducting liquid containing dissolved reactive substance. The reaction is considered to take place at the metal solution interface, due to the heterogeneity on the metal surface, which creates local anodic and cathodic sites on the metal. Environmental effects such as those of presence of oxygen and other oxidizers, changes in flow rates (velocity), temperature, reactant concentrations and ph would influence rates of anodic and cathodic reactions. Dry corrosion occurs in the absence of aqueous environment, usually in the presence of gases and vapours, mainly at high temperatures. Electrochemical nature of corrosion can be understood by examining zinc dissolution in dilute hydrochloric acid. Zn + 2HCl = ZnCl2+ H2 Anodic reaction is Zn = Zn e with the reduction of 2H + + 2e = H2 at cathodic areas on the surface of zinc metal. There are two half reactions constituting the net cell reaction. 15

16 Basic Causes of Corrosion Conditions necessary for corrosion [9]: For the purpose of this manual, electrochemical corrosion is the most important classification of corrosion. Four conditions must exist before electrochemical corrosion can proceed: 1- There must be something that corrodes (the metal anode). 2- There must be a cathode. 3- There must be continuous conductive liquid path (electrolyte, usually Condensate and salt or other contaminations). 4- There must be a conductor to carry the flow of electrons from the anode to the cathode. This conductor is usually in the form of metal-to-metal Contact such as in bolted or riveted joints. The elimination of any one of the four conditions will stop corrosion. Effect of material selection One of the fundamental factors in corrosion is the nature of the material. Materials are usually selected primarily for structural efficiency, and corrosion resistance is often a secondary consideration in design. Water intrusion Water intrusion is the principal cause of corrosion problems encountered in the field use of equipment. Water can enter an enclosure by free entry, capillary action, or condensation. With these three modes of water entry acting and with the subsequent confinement of water, it is almost certain that any enclosure will be susceptible to water intrusion. 16

17 Environmental factors The environment consists of the entire surrounding in contact with the material. The primary factors to describe the environment are the following: (a) Physical state gas, liquid, or solid. (b) Chemical composition constituents and concentrations. (c) Temperature. At normal atmospheric temperatures the moisture in the air is enough to start corrosive action. Oxygen is essential for corrosion to occur in water at ambient temperatures. Other factors that affect the tendency of a metal to corrode are: 1- Acidity or alkalinity of the conductive medium (ph factor). 2- Stability of the corrosion products. 3- Biological organisms (particularly anaerobic bacteria). 4- Variation in composition of the corrosive medium. 17

18 Mechanism of corrosion: Particularly under the broad definition of corrosion as the deterioration of materials by reaction with the environment, the number of mechanisms whereby deterioration occurs is large. What do we mean by mechanism of corrosion reaction [10]? We mean the behavior of a metal, or the way a metal reacts with an environment. This behavior may be simple and consist of one stage, for example, corrosion of iron in the oxygen atmosphere at high temperature. The corrosion reaction may be more complicated and consist of two and more stages, for example, when iron comes into contact with water or with hydrochloric acid. If two dissimilar metals, iron and zinc, contact together in salt water or metal are under stress in some environment, the corrosion reaction may be more complicated. In corrosion, of course, this rate should be as slow as possible. Because these processes cannot be observed directly on an atomic scale, it is necessary to infer possible mechanisms from indirect measurements and observations. In general, a mechanism of corrosion is the actual atomic, molecular, or ionic transport process that takes place at the interface of a material. These processes usually involve more than one definable step, and the major interest is directed toward the slowest step that essentially controls the rate of the overall reaction. All corrosion reactions are described by two mechanisms: 1. Non-Electrolytes (without the formation of electric current). 2. Electrolytes (with the formation of electric current and potential). 18

19 *Corrosion of metals by non-electrolytes: Non-electrolytes exist in gaseous (O2, Cl2), liquid (Br2) and solid (sugar) states. If metals come into contact with any dry gas or liquid non-electrolyte, corrosion (chemical reaction) occurs in one stage. Metals give their outer electrons to nonmetals (O2, Cl2, Br2 or S8) and are oxidized in one reaction: 2Fe (s) + 3Cl 2 (g) 2Fe (s) + 3Br 2 (l) 2FeCl 3 (s) 2FeBr 3 (s) 8Fe (s) + S 8 (l) 8FeS (s) Metals contacting with non-electrolytes do not have electric potential at their surfaces. Usually, such corrosion reactions occur under dry conditions: without water or, more precisely, without electrolyte. Characteristic feature of this corrosion mechanism is the absence of electric current and electric potential on the metal surface during corrosion. *Corrosion of metals in the presence of electrolytes: Electrochemical corrosion is a process occurring between a metal and the electrolyte environment not in one electrochemical reaction, and the corrosion rate depends on the electric potential on the metal surface. 1-Anodic Reactions: The loss of metal occurs as an anodic reaction. Examples are: Where the notations (s), (aq), and (l) refer to the solid, aqueous, and liquid phases, respectively. 19

20 Each of the above reactions in equations. (1), (2), and (3) is an anodic reaction because of the following: (1) A given species undergoes oxidation, i.e., there is an increase in its oxidation number. (2) There is a loss of electrons at the anodic site (electrons are produced by the reaction). The following reaction is also an anodic reaction: The oxidation number of the Fe species on the left, i.e., in the ferrocyanate ion, is +2, and the oxidation number of Fe in the ferricyanate ion on the right is +3. Thus, there is an increase in oxidation number. In addition, electrons are produced in the electrochemical half-cell reaction, so Eq. (4) is an anodic reaction. By the same reasoning the following is also an anodic reaction: Although Eqs. (4) and (5) are anodic reactions, they are not corrosion reactions. There is a charge transfer in each of the last two equations, but not a loss of metal. Thus, not all anodic reactions are corrosion reactions. This observation allows the following scientific definition of corrosion: Corrosion is the simultaneous transfer of mass and charge across a metal/solution interface. 20

21 2- Cathodic Reactions: An example of a cathodic reaction is the reduction of two hydrogen ions at a surface to form one molecule of hydrogen gas: This is an anodic reaction because of the following: (1) A given species undergoes reduction, i.e., there is a decrease in its oxidation number. (2) There is a gain of electrons at the cathodic site (electrons are consumed by the reaction)

22 3- Coupled Electrochemical Reactions: On a corroding metal surface, anodic and cathodic reactions occur in a coupled manner at different places on the metal surface. The behavior for an iron surface immersed in an acidic aqueous environment. At certain sites on the iron surface, iron atoms pass into solution as Fe 2+ ions. The two electrons produced by this anodic half-cell reaction are consumed elsewhere on the surface to reduce two hydrogen ions to one H2 molecule. The reason that two different electrochemical half-cell reactions can occur on the same metal surface lies in the heterogeneous nature of a metal surface. Polycrystalline metal surfaces contain an array of site energies due to the existence of various crystal faces (i.e., grains) and grain boundaries. In addition, there can be other defects such as edges, steps, kink sites, screw dislocations, and point defects. 22

23 Moreover, there can be surface contaminants due to the presence of impurity metal atoms or to the adsorption of ions from solution so as to change the surface energy of the underlying metal atoms around the adsorbate. "Figure illustrates the coupled electrochemical reactions for an iron surface immersed in a neutral or a basic aqueous solution. Metal atoms at the highest energy sites are most likely to pass into solution. These high-energy sites include atoms located at the edges and corners of crystal planes, for example. Stressed surfaces also contain atoms that are reactive because they have a less stable crystalline environment. When a metal is cold worked or shaped, the metal lattice becomes strained, and atoms located in the strained regions tend to go into solution more readily than do atoms in unstrained regions. Once the process of metal dissolution process begins, a new energy distribution of sites is established. Then, the positions of anodic and cathodic surface sites change randomly with time so that the overall effect is uniform corrosion of the metal. 23

24 The overall chemical reaction is thus the sum of the two half-cell reactions: At the local anodes Fe (s) Fe 2+ (aq) + 2e At the local cathodes 2H + (aq) + 2e H 2 (g) The overall reaction is the sum of these two half-cell reactions: Fe (s) + 2H + (aq) Fe 2+ (aq) + H 2 (g) An electrolyte is a solution which contains dissolved ions capable of conducting a current. The most common electrolyte is an aqueous solution, i.e., water containing dissolved ions; but other liquids, such as liquid ammonia, can function as electrolytes. Electrochemical Polarization: Electrochemical polarization (usually referred to simply as polarization ) is the change in electrode potential due to the flow of a current. There are three types of polarization [11]: (1) Activation polarization is polarization caused by a slow electrode reaction. (2) Concentration polarization is polarization caused by concentration changes in reactants or products near an electrode surface. (3) Ohmic (resistance) polarization is polarization caused by IR drops in solution or across surface films, such as oxides (or salts). The degree of polarization is defined as the over voltage (or over potential) η given by the following equation: Where E is the electrode potential for some condition of current flow and E o is the electrode potential for zero current flow. 24

25 Anodic and Cathodic Polarization: Either an anode or a cathode can be polarized: Anodic polarization is the displacement of the electrode potential in the positive direction so that the electrode acts more anodic. Cathodic polarization is the displacement of the electrode potential in the negative direction so that the electrode acts more cathodic. 25

26 Potential ph Diagrams: Potential ph diagrams, also known as Pourbaix diagrams [12], are graphical representations of the stability of a metal and its corrosion products as a function of the potential and ph (acidity or alkalinity) of the aqueous solution. PH is an important variable of aqueous solutions, and it affects the equilibrium potentials of a majority of the possible reactions that can occur. On this basis, Marcel Pourbaix derived and presented his ph potential diagrams, also called equilibrium diagrams because these diagrams apply to conditions where the metal is in equilibrium with its environment. These diagrams have become an important tool for the illustration of the possibilities of corrosion. Pourbaix diagrams are available for over 70 different metals. These diagrams indicate certain regions of potential and ph where the metal undergoes corrosion and other regions of potential and ph where the metal is protected from corrosion. *In a Pourbaix diagram, there are three possible types of straight lines: 1-Horizontal lines, which are for reactions involving only the electrode potential E (but not the ph). 2-Vertical lines, which are for reactions involving only the ph (but not the electrode potential E). 3-Slanted lines, which related to reactions involving both the electrode potential E and the PH. Pourbaix diagrams also contain regions or fields between the various lines where specific chemical compounds or species are thermodynamically stable. 26

27 *Pourbaix diagram regions: 1-When the stable species is a dissolved ion, the region on the Pourbaix diagram is labeled as a region of corrosion. 2-When the stable species is either a solid oxide or a solid hydroxide, the region on the Pourbaix diagram is labeled as a region of passivity, in which the metal is protected by a surface film of an oxide or a hydroxide. 3-When the stable species is the unreacted metal species itself, the region is labeled as a region of immunity. Pourbaix diagrams for various metals and metalloids arranged after their nobleness 27

28 Example: Potential ph (Pourbaix) diagram for Fe H2O system. Figure: Pourbaix diagram for Iron In the diagram, the horizontal lines represent pure electron transfer reactions dependent solely on potential, but independent of ph: 1) Fe =Fe e 2) Fe 2+ =Fe 3+ + e These lines extend across the diagram until the ph is sufficiently high to facilitate the formation of hydroxides, represented by vertical lines,thereby reducing the concentration Fe 2+ and Fe 3+ ions. 28

29 The boundary is often set arbitrarily at the concentration of these ions at 10 6 g-ions/liter,which is indicative of a negligible dissolution or corrosion of the metal in the medium. The vertical lines in the Figure correspond to the reactions: There is no electron transfer involved and the reactions are solely dependent on ph. The sloping lines in Figure 2.3 represent equilibria involving both electron transfer and ph; for example: The hydrogen and oxygen are also shown in the diagram by the dotted lines. The hydrogen line represents the equilibria: 2H + + 2e = H in acid solutions Or 2H 2 O+2e + = H 2 2OH in neutral or alkaline solutions" These two reactions are equivalent and their ph dependence of single electrode potential is represented by: At ph = 0; that is, for [H + ] = 1, E O H+/H2 = 0 and the slope is 0.059V. Similarly,for oxygen equilibrium with water the corresponding reactions at lower and higher ph are: and 29

30 The ph dependence of single electrode potential is represented by: At ph = 0, E O O2 /H2O =1.226 V and at ph = 1, (i.e., for [OH ] = 1), E O O2 /H2O = V. Here again, the slope of the line is 0.059V. Water is stable in the area designated by these two lines. Below the hydrogen line it is reduced to hydrogen gas, and above the oxygen line it is oxidized to oxygen. The potential ph diagram shows three clear-cut zones: 1. Immunity zone: Under these conditions of potential and ph, iron remains in metallic form. 2.Corrosion zone: Under these conditions of potential and ph, iron corrodes, forming Fe 2+ or Fe 3+ or HFeO Passive zone: Under these conditions of potential and ph, protective layers of Fe(OH) 3 form on iron and further corrosion of iron does not take place. *Uses of Pourbaix diagram 1. Predicting the spontaneous direction of reactions 2. Estimating the stability and composition of corrosion products. 3. Predicting environmental changes that will prevent or reduce Corrosion. 30

31 Forms of Corrosion: Corrosion occurs in several widely differing forms [13]. Classification is usually based on one of three factors: *Nature of the corrodent: Corrosion can be classified as wet or dry. A liquid or moisture is necessary for the former, and dry corrosion usually involves reaction with high-temperature gases. *Mechanism of corrosion: This involves either electrochemical or direct chemical reactions. *Appearance of the corroded metal: Corrosion is either uniform and the metal corrodes at the same rate over the entire surface, or it is localized, in which case only small areas are affected. Classification by appearance, which is particularly useful in failure analysis, is based on identifying forms of corrosion by visual observation with either the naked eye or magnification. The morphology of attack is the basis for classification. Forms of wet (or aqueous) corrosion can be identified based on appearance of the corroded metal. These are: General (uniform) corrosion Galvanic (bi-metallic) corrosion Crevice corrosion Pitting corrosion Erosion corrosion Stress corrosion cracking Biological corrosion Selective leaching Liquid metal attack Filiform corrosion 31

32 General (Uniform) corrosion: It is a very common form found in ferrous metals and alloys that are not protected by surface coating or inhibitors. A uniform layer of rust on the surface is formed when exposed to corrosive environments Atmospheric corrosion is a typical example of this type. Galvanic corrosion: Galvanic corrosion is a chemical or an electrochemical corrosion. It is due to a potential difference between two different metals connected through a circuit for current flow to occur from more active metal (more negative potential) to the more noble metal (more positive potential), where the active one corrodes. Eg: - Copper containing precipitates in aluminium alloys. Impurities such as iron and copper in metallic zinc. Crevice corrosion: It is a localized attack on a metal adjacent to the crevice between two joining surfaces (two metals or metal-nonmetal crevices). The corrosion is generally confined to one localized area to one metal. This type of corrosion can be initiated by concentration gradients (due to ions or oxygen). Accumulation of chlorides inside crevice will aggravate damage. 32

33 Pitting corrosion: It is a localized phenomenon confined to smaller areas. Formation of micropits can be very damaging. In pitting corrosion the surface of the metal is attacked in small-localized areas. Organisms in water or breaks in a passive film can initiate corrosion. In pitting corrosion very little metal is removed from the surface but the effect is marked. Pitting corrosion is characterized by the following features: 1. The attack is spread over small discrete areas. Pits are sometimes isolated and sometimes close together, giving the area of attack a rough appearance. 2. Pits usually initiate on the upper surface of the horizontally placed parts and grow in the direction of gravity. 3. Pitting usually requires an extended initiation period before visible pits appear. 4. Conditions prevailing inside the pit make it self-propagating without any external stimulus. Once initiated, the pit grows at an ever-increasing rate. 5. Stagnant solution conditions lead to pitting. 6. Stainless steels, aluminum, and their alloys are particularly susceptible to pitting. Carbon steels are more resistant to pitting than stainless steels. 33

34 Erosion corrosion: The term erosion applies to deterioration due to mechanical force. Erosion is the removal of metal by the movement of fluids against the surface. Depending on the rate of this movement, abrasion takes place. The combination of erosion and corrosion can provide a severe rate of corrosion. This type of corrosion is characterized by grooves and surface patterns having directionality. Typical examples are: Stainless alloy pump impeller, Condenser tube walls. Stress corrosion cracking: Structural parts subjected to a combination of a tensile stress and a corrosive environment may prematurely fail at a stress below the yield strength. This phenomenon is known as environmentally induced cracking (EIC). selective leaching: The removal of one of the components of an alloy by corrosion is termed selective leaching. Dezincification is the term used to describe the leaching of zinc from brass, which is the most common example of selective leaching. The less noble component of an alloy is usually the element that is removed, such as zinc in brass. 34

35 Liquid metal attack: Metallic components may come in contact with liquid metal during operations such as brazing, soldering, or galvanizing and in some applications such as the use of molten sodium as a coolant in fast-breeding nuclear reactors. Liquid metal may corrode the solid metal component or there may be diffusion-controlled intergranular penetration of liquid metal in the solid metal. Filiform corrosion: It is a special type of crevice corrosion. Metals with semipermeable coatings or films may undergo a type of corrosion resulting in numerous meandering threadlike filaments of corrosion beneath the coatings or films. 35

36 Biological Corrosion: Biologically influenced corrosion or Microbial-induced corrosion (MIC) refer to [14] the degradation of metals caused by the activity of living organisms. Contributing to the corrosion are both micro- and macroorganisms in a variety of environments, including domestic and industrial fresh waters, soils, groundwater, seawater, natural petroleum products, and oil-emulsion cutting fluids. Macroorganisms: [15] A wide variety (about 2000 species) of larger organisms, primarily marine plants and animals, have been associated with the fouling of metals in sea water. The principal fouling animals include: barnacles, tubeworms, bryozoa (polyzoa), hydroids, mussels, and tunicates (sea squirts). The principal plants are algae. A series of well-illustrated catalogues of these organisms is being published under the auspices of the Organization for Economic Cooperation and Development [16]. Macrobiological organisms are also capable of causing corrosion as well as fouling. In most cases, fouling presents more of a problem than corrosion. Because these organisms remain attached to the metal surface, their accumulation on the bottom of a ship s hull increases the drag and power requirement. Such accumulations in heat exchangers impair heat transfer and fluid flow, while in pipelines they may clog the pipeline as well as impair fluid flow. The metabolic by-products of these organisms are often acidic and therefore corrosive. In addition, the anaerobic conditions underneath the macroorganisms can favor the growth of anaerobic bacteria, which in turn accelerates the corrosion of the metal. 36

37 Microorganisms[17] are usually considered to be organisms so small that they can be seen only with the aid of a light or electron microscope. The term microorganism covers a wide variety of life forms, including bacteria, bluegreen cyanobacteria, algae, lichens, fungi, and protozoa. All microorganisms may be involved in the bio-deterioration of metals. They vary greatly in their nutritional requirements and tolerances to heat, light, ph, oxygen, moisture, etc. Most of the microorganisms involved in MIC are chemolithotrophs and can be aerobic anaerobic, mesophilic thermophilic, autotrophs -heterotrophs, acidophilic- neutrophilic and many are slime formers. Chemotrophs get energy from chemical sources unlike photosynthetic organisms. Microorganisms associated with MIC are generally characterized by a number of features such as: [18] Small size (few micrometers) Ubiquitous and omnipotent Sessile or motile (active or sedentary) Ability to attach to substrates and grow colonies. Extremophiles (tolerant to wide range of metal concentrations, acidity, temperature, pressure, oxygen and lack of oxygen) Rapid reproduction. Generate organic and inorganic acids, alkalis, and extracellular polymeric substances such as proteins and polysaccharides. Can oxidize or reduce metals and ions. 37

38 Bacteria are small, unicellular organisms which reproduce by fission. The cells exist in anyone of three basic shapes: rods, curved or spiral rods, and spheres. Bacterial cells possess a cell wall, a cytoplasmic membrane, nuclear material, and various types of inclusion bodies. Surrounding the cell wall may be a slime layer and often a capsule. Motile bacteria have appendages known as flagella which usually serve as a means of propulsion for the cell. In addition, some bacterial cells (primarily certain rod forms) form an endospore (spore with a cell).there is wide diversity with regard to their metabolisms. An important feature of microbial life is the ability to degrade any naturally occurring compound. They are classified as to their source of metabolic energy as follows: 38

39 In addition to energy and carbon sources, nitrogen, phosphorus, and trace elements are needed by microorganisms[19]. Nitrogen compounds may be inorganic ammonium nitrate as well as organically bound nitrogen (e.g., amino acids, nucleotides). With the help of an enzyme called nitrogenase, bacteria are able to fix nitrogen from atmospheric nitrogen, producing ammonia that is incorporated into cell constituents. Phosphorus is taken in as inorganic phosphate or as organically bound phosphoroxylated compounds such as phosphorus-containing sugars and liquids. Phosphorus, in the form of adenosine triphosphate (ATP), is the main energy-storing compound. For many of the metabolic purposes, trace elements are needed. Cobalt aids in the transfer of methyl groups from/to organic or inorganic molecules. Vitamin B12, cobalamin, is involved in the methylation of heavy metals such as mercury. Iron, as Fe +2 or Fe +3 is required for the electron transport system, where it acts as an oxidizable/reducible central atom in cytochrome of non hemo-iron-sulfur proteins. Those organisms living with the amount of oxygen contained in the air are called aerobes, whereas those that perform their metabolism without any trace of oxygen are called anaerobes. The latter are able to use bound oxygen (sulfate, carbon dioxide) or to ferment oxygen compounds. 39

40 The methods by which microorganisms increase the rate of corrosion of metals and/or their susceptibility to localized corrosion in an aqueous environment are: 1. Production of metabolites. Bacteria may produce organic acids, inorganic acids, sulfides, and ammonia, all of which may be corrosive to metallic materials. 2. Destruction of protective layers. Organic coatings may be attacked by various microorganisms, leading to the corrosion of the underlying metal. 3. Hydrogen embrittlement. By acting as a source of hydrogen and/or through the production of hydrogen sulfide, microorganisms may influence hydrogen embrittlement of metals. 4. Formation of concentration cells at the metal surface and, in particular, oxygen concentration cells. A concentration cell may be formed when a biofilm or bacterial growth develops heterogeneously on the metal surface. Some bacteria may tend to trap heavy metals such as copper and cadmium within the extracellular polymeric substance, causing the formation of ionic concentration cells. These lead to localized corrosion. 5. Modification of corrosion inhibitors. Certain bacteria may convert nitrite corrosion inhibitors used to protect aluminum and aluminum alloys from nitrate and ammonia. 6. Stimulation of electrochemical reactors. An example of this type is the evolution of cathodic hydrogen from microbially produced hydrogen sulfide. 40

41 Microbial-induced corrosion (MIC) can result from: 1. Production of sulfuric acid by bacteria of the genus Thiobacillus through the oxidation of various inorganic sulfur compounds; the concentration of sulfuric acid may be as high as 10 to 12%. 2. Production of hydrogen sulfide by sulfate-reducing bacteria. 3. Production of organic acids. 4. Production of nitric acid. 5. Production of ammonia. Biologically influenced corrosion does not represent a special form of corrosion but rather the aggravation of corrosion under environmental conditions in which corrosion rates are expected to be low. Corrosive conditions can be developed by living microorganisms as a result of their influence on anodic and cathodic reactions. The metabolic activity can directly or indirectly cause deterioration of a metal by the corrosion process; this activity can: 1. Produce a corrosive environment. 2. Create electrolytic cells on the metal surface. 3. Alter the resistance of surface films. 4. Have an influence on the rate of anodic or cathodic reactions. 5. Alter the environmental composition. 41

42 Mechanism of biological corrosion: [20] Microbial corrosion can occur and advance through two main mechanisms. The first method of microbially enhanced corrosion occurs from microorganisms producing acidic metabolic by-products or from microorganisms participating directly in the electrochemical corrosion of the pipe. When it is suspected that a material failure was caused by microbial corrosion, it is reasonable to ask: How do we know that the corrosion process was influenced by microorganisms? To address this question, many research groups have attempted to find a fingerprint of microbially influenced corrosion (MIC). Despite significant research effort, no such fingerprint characteristic of MIC has yet been found, and there are good reasons to believe that a universal mechanism of microbially stimulated corrosion does not exist. Instead of a universal mechanism, several mechanisms by which microorganisms affect the rates of corrosion have been described, and the diversity of these mechanisms is such that it is difficult to expect that a single unified concept can be conceived to bring them all together. From what we now understand, and what has been demonstrated by numerous researchers, accelerated corrosion of metals in the presence of microorganisms stems from microbial modifications to the chemical environment near metal surfaces. 42

43 An important aspect of quantifying mechanisms of microbially influenced corrosion is to demonstrate how the microbial reactions interfere with the corrosion processes and, based on this, identify products of these reactions on the surfaces of corroding metals using appropriate analytical techniques. The existence of these products, associated with the increasing corrosion rate, is used as evidence that the specific mechanism of microbially influenced corrosion is active. There is no universal mechanism of MIC. Instead, many mechanisms exist and some of them have been described and quantified better than other. Therefore, it does not seem reasonable to search for universal mechanisms, but it does seem reasonable to search for evidence of specific, well-defined microbial involvement in corrosion of metals. Microorganisms involved in MIC can be generally classified as: a) Sulfate-reducing bacteria (SRB). b) Iron-reducing bacteria (IRB). Because microbial induced corrosion (MIC) gives the appearance of pitting, it is first necessary to diagnose the presence of bacteria. 43

44 Sulfate Reducing Bacteria (SRB): Sulfate-reducing bacteria (SRB) [21] are a group of the most frequent causes for bio-corrosion. Common SRB include Desulfovibrio, Desulfobacter and Desulfotomaculum. SRBs can grow in soil, fresh water and seawater environments and also in stagnant areas. Tolerate ph ranges Oil, gas and shipping industries are seriously affected by SRB activities (soil and water) due to H 2 S generation. They oxidize organic substances to organic acids or CO 2, by reduction of sulfate to hydrogen sulfide which reacts with metals to produce metal sulfides as corrosion products through anaerobic respiration. Oxygen depletion at the surface also provides a condition for anaerobic organisms like sulfate-reducing bacteria to grow. Aerobic bacteria near the outer surface of the biofilm consume oxygen and create a suitable habitat for the sulfate reducing bacteria at the metal surface. Symptoms of SRB-influenced corrosion are hydrogen sulfide (rotten egg) odor, blackening of waters, and black deposits. The black deposit is primarily iron sulfide. One way to limit SRB activity is to reduce the concentration of their essential nutrients: phosphorus, nitrogen, and sulfate. Sulfate reducing bacteria under microscope 44

45 Role of SRB in metallic corrosion can be understood by [22]: a) H 2 S generation b) Creation of oxygen concentration cells c) Formation of insoluble metal sulfides d) Cathodic depolarization Characteristics of some sulfate reducing bacteria relevant to MIC are given in the following Table: 45

46 Microscopic images from three sulfate-reducing bacteria obtained from sediments of IODP Site U1301. (A,B) Desulfovibrio aespoeensis strain; (C,D) Desulfovibrio indonesiensis strain; (E,F) Desulfotignum 46

47 *Corrosion Mechanism Involving sulfate-reducing bacteria (SRB): Perhaps the best-known mechanism of MIC involves corrosion cells generated and sustained on steel surfaces by the action of anaerobic SRB. These organisms reduce sulfate to sulfide in their metabolism and are commonly found in mixed microbial communities present in soils and natural waters. Kuhr and Vluglt [23] proposed the classical theory of cathodic depolarization of SRB corrosion, it is the main mechanism of SRB corrosion, it is believed that under hypoxic conditions, SRB cathodic depolarization effect to remove the hydrogen atoms from the metal surface, so that the corrosion process continue. Reaction is as follows: 4Fe 4Fe 2+ +8e (Anodic reaction) 8H 2 O 8H + +8OH - 8H + + 8e 8H 2- SO 4 + 8H S H 2 O Fe 2+ + S 2- FeS 3Fe OH - 3Fe(OH) 2 (Cathode reaction) (SRB cathodic depolarization) (Corrosion products) (Corrosion products) The total equation is: 4Fe 2+ + SO H 2 O FeS + 3Fe(OH) 2 + 2OH - 47

48 In industrial systems, [24] biodegradable materials, such as some of the hydrocarbons found in oil and gas operations or susceptible components of coating materials, can provide a source of nutrients for microbial growth. Cathodic hydrogen formed on a metal surface by active corrosion or by cathodic protection (CP) can specifically promote growth of organisms, including SRB that are able to use hydrogen in their metabolism. Severe corrosion cells develop as sulfide, produced by the microbial reduction of sulfate, combines with ferrous ions, released by the corrosion process, to produce insoluble black iron sulfides: Consistent with the importance of this corrosion process in industrial facilities, commercial test kits have been developed for enumerating or assessing the activity of SRB in operating systems. Customers complain relating to offensive odors at isolated points in a distribution system are often attributable to sulfate-reducing bacteria 48

49 This figure illustrates a plausible mechanism base on a galvanic couple formed between iron and iron sulfide sustained and extended by the active involvement of SRB. The way in which electrons are transferred from iron sulfide to the SRB, for example, is not well resolved. It may occur directly or via formation of cathodic hydrogen, as shown in the previous figure, or by another reaction involving reduction of H 2 S. Typical rates of metal loss for unprotected line pipe steel in an SRB/FeS corrosion scenario are 0.2 mm/year for general corrosion and 0.7 mm/year for pitting corrosion, but the corrosion rate observed depends on the concentration of FeS. 49

50 Iron Reducing Bacteria (IRB) : The iron reducing bacteria (IRB) or Fe-reducing bacteria (FeRB) [25] can grow either in cold temperatures or in mesophilic and thermophilic conditions. The iron reducers prefer neutral PH, present in water, with oxygen. In natural systems, Fe +3 minerals can be microbiologically reduced by strictly aerobic iron-reducing bacteria (IRB) using a wide range of organic compounds as electron donors or by using H 2. The role of iron reducing bacteria in MIC is so inseparable from the role of oxygen. iron bacteria has the capable of oxidation of Fe 2+ to Fe 3+ ions and use energy to grow, eventually form Fe(OH) 3 precipitation. Studies suggest that[26], iron bacteria mainly take part in the corrosion in the form of corrosion scale and in a short time to produce a large number of iron oxide deposition. The corrosion of iron bacteria occurs through the crevice corrosion mechanism. 2Fe 2Fe e O 2 + 2H 2 O + 4e 4OH 2Fe OH 2Fe(OH) 2 4Fe(OH) 2 + O 2 + 2H 2 O 4Fe(OH) 3 (Anodic reaction) (Cathode reaction) (Corrosion products) (Corrosion products) The total equation is: 4Fe+ 3O2 + 6H2O 2Fe(OH)3 50

51 Prevention of biological corrosion: There are many approaches that can be used to prevent or to minimize MIC. Before any remedial action can be taken, it is necessary to identify the type of bacteria involved in the corrosion. Among the choices are: 1. Selection and Modification of Environment: [27] As natural environments vary so widely in their corrosive properties, selection or modification of the environment so as to render it less corrosive should be the first consideration in corrosion prevention, where possible. Obviously, utilization of this principle affords greater promise of avoiding severe corrosion when metal structures are to be placed in the soil rather than in an aqueous environment. The environment may be changed in the following ways in order to reduce corrosion rates: Decreasing or increasing the temperature, the flow velocity, and the content of oxygen. Mitigation of biological corrosion has been affected by changing the environment to a less corrosive one in a number of cases. When placing of metal structures in localized areas of anaerobic soil cannot be avoided, it may be possible to provide aerobic conditions (air is the cheapest inhibitor for sulfate-reducers) by surrounding the structure with gravel, providing there is adequate drainage. Aeration of water in a closed recirculating system reduces the activity of anaerobic bacteria 51